In recent years, optical communications is rapidly developed by virtue of having widely available frequency bands and advantages of being light weight and being free of electromagnetic interference.
In a high-speed optical transmission system, a multi-system modulation technology is frequently used with forward error correction coding at a transmitter end to improve transmission efficiency of the system and lower bit error rate of the system. However, when coherent demodulation is performed on a signal at a receiver end, a carrier recovered by the receiver end may have a same frequency and phase as a carrier of a received signal, or may have a same frequency as and a reverse phase to a carrier of the received signal. When the carrier recovered by the receiver end has the same frequency as and the reverse phase to the carrier of the received signal, a data stream polarity after demodulation is reverse to a data stream sending polarity, which severely affects system performance.
In the prior art, the transmitter end prevents, by using a differential coding method, the data stream polarity obtained after the receiver end performs demodulation from being reverse to the data stream sending polarity, that is, the transmitter end transmits information by using a phase difference between adjacent symbols, and the receiver end performs decoding by using a differential decoding method.
However, in the prior art, bit error spread may be caused in a differential decoding process, leading to a relative increase of an input bit error rate of a forward error correction module of the receiver end.